The present invention broadly relates to the autocatalytic chemical deposition of nickel, and more particularly, to an improved aqueous electroless nickel plating bath and process for depositing nickel on a substrate.
A variety of nickel containing aqueous solutions have heretofore been used or proposed for use for chemically depositing nickel on a substrate incorporating various additive components for controlling the rate of nickel deposition and for promoting stability of the bath after prolonged usage. Among such compositions are those such as disclosed in U.S. Pat. Nos. 2,762,723; 2,822,293; and 3,489,576. In addition to a controlled concentration of nickel ions, such prior art electroless nickel plating baths conventionally employ hypophosphite anions for reducing the nickel cation to the metallic state and the hypophosphite anions are in turn oxidized to phosphite anions and other degradation products some of which combine with other nickel ions present in the solution forming a finely particulated dispersion producing a random chemical reduction of the other nickel ions present in the bath causing the resultant nickel deposit on the substrate to become progressively coarse, rough and sometimes porous. The presence of such fine-sized dispersed particulate matter also promotes instability of the chemical balance of the bath ultimately resulting in a decomposition thereof necessitating discarding the bath and replacement.
For these and other reasons, various additive agents as described in the aforementioned U.S. patents have heretofore been employed or proposed for use to stabilize the bath and to further control the rate of nickel deposition on a substrate being plated. In such electroless nickel plating baths employing hypophosphite ions as the reducing agent, the nickel deposit actually comprises an alloy of nickel and phosphorus with the phosphorus content usually ranging from about 2 to about 15 percent by weight. The physical and chemical properties of such nickel-phosphorus alloy deposits are related to the percentage of the phosphorus present and in turn, the percentage of phosphorus in the deposit is influenced by a number of factors including the bath operating temperature, the operating pH, the hypophosphite ion concentration, the nickel ion concentration, the phosphite ion and hypophosphite degradation product concentration as well as the total chemical composition of the bath including additive agents.
In end uses of electroless nickel plated articles, those applications requiring maximum deposit hardness or nickel deposits which are nonmagnetic, it is normally necessary to provide nickel alloy deposits with a relatively high percentage of phosphorus such as 9 percent by weight or greater. However, there are numerous other applications for electroless nickel-phosphorus alloys in which a lower percentage of phosphorus is desirable and an Aerospace Material Specification, AMS2405A provides for nickel-phosphorus alloy deposits in which the phosphorus content is to be held to a minimum and, in any event, shall not exceed 8 percent by weight.
Prior art compositions and processes for producing nickel-phosphorus alloy deposits having low percentages of phosphorus have been found susceptible to producing bath instability, a shortening of the operating life of the bath and/or have caused increased difficulty to control the bath because of the relatively narrow concentration ranges of some of the bath constituents. For example, the addition of thiourea to an electroless nickel bath has been found effective to reduce the phosphorus content in the resultant nickel deposit. However, at a concentration of between 2.5 and 3 parts per million (33 to 40 micro mol per liter), thiourea causes such bath formulations to cease plating. It has been reported that the critical narrow concentration limits of thiourea in an electroless nickel plating bath to provide satisfactory operation renders this additive agent impractical for commercial plating installations because analysis and replenishment of such baths to maintain proper composition parameters is difficult, time consuming and cost intensive.
Alternative sulfur-containing organic additive agents have been proposed for stabilizing and/or increasing the deposition rate of nickel from electroless nickel plating baths such as described in U.S. Pat. Nos. 2,762,723 and 3,489,576. Such alternative additive materials have also been found commercially impractical because of a very narrow useful concentration range and moreover, many of such sulfur-containing organic compounds do not produce a nickel-phosphorus alloy deposit in which the phosphorus content is below about 8 percent by weight.
Prior art compositions and processes for producing nickel-phosphorus alloy deposits of relatively high phosphorus contents have also been subject to the disadvantages of requiring relatively rigid control of the concentration of the bath constituents detracting from the ease of control, maintenance and replenishment of such baths to maintain optimum operating performance. The use of stabilizing agents for providing increased bath stability has occasioned in prior art compositions a condition of over stabilization whereby a cessation of plating occurs. In such instances, it has been necessary to discard the bath and prepare a new operating bath which constitutes a costly and time-consuming operation.
The present invention provides for an improved electroless nickel plating bath and process for depositing a nickel-phosphorus alloy of relatively low phosphorus content incorporating an additive agent which can satisfactorily be employed over a relatively broad operating concentration range while at the same time increasing the rate of deposition of the nickel by as much as 30 percent or more. The present invention further provides for an improved electroless nickel plating bath and process suitable for use in depositing nickel-phosphorus platings of relatively high phosphorus content providing for greater latitude in variations in the bath constituents thereby achieving simpler control and facilitating maintenance and replenishment of the bath. The present invention further contemplates a method for rejuvenating or restoring an electroless nickel plating bath which has been rendered inoperative due to over stabilization thereof by inclusion of organic and/or inorganic stabilizing agents in excessive amounts by the addition of an additive agent of the present invention whereby satisfactory operation of the bath is restored.